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121.
刘贵 《变流技术与电力牵引》2005,(5):32-36
介绍了绘制正弦线电流的开关模式单相半控桥整流器,用它来获得功率因数补偿并保持直流环节电压的恒定.在该整流器中使用4个有源开关,以便在交流端电压上产生单极脉宽调制(PWM)电压波形.与中性点箝位变流器(NPC)相比,该整流器没有箝位二极管也能实现三点式PWM控制.控制电路采用两个控制回路:外部控制回路用比例积分电压控制器调整直流环节电压,采用相位闭环电路产生与电源电压同相的正弦波形来实现功率因数补偿.在内部控制回路中,用基于载波的电流控制器跟踪线电流信号.为补偿由于负载变化所引起的中点电压,控制电路采用中点电压补偿器.该整流器的交流侧可产生3个电压电平,并用计算机仿真与试验结果验证了控制算法的有效性. 相似文献
122.
Using CalCOFI data for coastal shallow stations (above 100 m depth), higher than expected nitrate concentrations were detected in near-surface high-temperature waters off of Central Baja California during some El Niño winters. Though recent data are not available for Central Baja California, past El Niño data, though limited, showed nitrate concentrations above 16 μM at temperatures above 16 °C, and nitrate concentrations between 1 and 2 μM at 19 °C, while the previously established relationship of temperature and nitrate for California Current waters predicts nitrate depletion above 14 or 15 °C. The anomalous, high temperature–high nitrate enrichment events documented in Central Baja California were detected as shallow as 9 m and as deep as 73 m, were associated with low-oxygen (between 2 and 4 ml/l) and high-salinity (between 33.8 and 34.3 psu) waters, and occurred during the winter months of an El Niño year. Using recent data for San Diego, CA, similar but weaker enrichment events were detected for the El Niño winter of 1997–1998. The periodic shoaling of a subsurface subtropical water mass of high temperature, high salinity, low oxygen and high nutrients during some El Niño winters is proposed to cause periodic enrichment and to maintain productivity during warming events in this area. Enrichment events were not detected off Ensenada, in Northern Baja California, possibly due to the amplification of the onshore flow during El Niño there, or due to the Ensenada front. The proposed mechanism of periodic enrichment of nutrient-depleted surface waters during some El Niño winters by subsurface waters from the California Undercurrent may explain the following: (1) survival of giant kelp forests at their southern limit in Central Baja California documented during past El Niño events in warm waters, (2) the rapid recovery and high carrying capacity of giant kelp documented after the mass disappearance during El Niño 1997–1998, and (3) the increase in the extent of mesotrophic chlorophyll detected in the area during the 1997–1998 and 1982–1983 El Niño events. 相似文献
123.
Peter B. Marlow 《Maritime Policy and Management》1991,18(4):283-311
This paper concentrates on the determinants of investment and examines both theoretical and empirical work on the investment function. The major determinants are considered and a case study of the United Kingdom shipping industry in the period 1963-1987 is undertaken. The analysis is unable to find any real link between the value of the investment incentives packages available to the UK shipping industry and the size of the UK fleet during this period. The conclusion follows that while financial factors (e.g. favourable credit arrangements) or individual components (e.g. investment grants) of fiscal packages may influence the decision to invest, the total package has not affected the level of shipping investment in the manner expected. 相似文献
124.
Lawrence Frank Mark Bradley Sarah Kavage James Chapman T. Keith Lawton 《Transportation》2008,35(1):37-54
The primary purpose of this study was to investigate how relative associations between travel time, costs, and land use patterns
where people live and work impact modal choice and trip chaining patterns in the Central Puget Sound (Seattle) region. By
using a tour-based modeling framework and highly detailed land use and travel data, this study attempts to add detail on the
specific land use changes necessary to address different types of travel, and to develop a comparative framework by which
the relative impact of travel time and urban form changes can be assessed. A discrete choice modeling framework adjusted for
demographic factors and assessed the relative effect of travel time, costs, and urban form on mode choice and trip chaining
characteristics for the three tour types. The tour based modeling approach increased the ability to understand the relative
contribution of urban form, time, and costs in explaining mode choice and tour complexity for home and work related travel.
Urban form at residential and employment locations, and travel time and cost were significant predictors of travel choice.
Travel time was the strongest predictor of mode choice while urban form the strongest predictor of the number of stops within
a tour. Results show that reductions in highway travel time are associated with less transit use and walking. Land use patterns
where respondents work predicted mode choice for mid day and journey to work travel.
Lawrence Frank is an Associate Professor and Bombardier Chair in Sustainable Transportation at the University of British Columbia and a Senior Non-Resident Fellow of the Brookings Institution and Principal of Lawrence Frank and Company. He has a PhD in Urban Design and Planning from the University of Washington. Mark Bradley is Principal, Mark Bradley Research & Consulting, Santa Barbara California. He has a Master of Science in Systems Simulation and Policy Design from the Dartmouth School of Engineering and designs forecasting and simulation models for assessment of market-based policies and strategies. Sarah Kavage is a Senior Transportation Planner and Special Projects Manager at Lawrence Frank and Company. She has a Masters in Urban Design and Planning from the University of Washington and is a writer and an artist based in Seattle. James Chapman is a Principal Transportation Planner and Analyst at Lawrence Frank and Company in Atlanta Georgia. He has a Masters in Engineering from the Georgia Institute of Technology. T. Keith Lawton transport modeling consultant and past Director of Technical services, Metro Planning Department, Portland, OR, has been active in model development for over 40 years. He has a BSc. in Civil Engineering from the University of Natal (South Africa), and an M.S. in Civil and Environmental Engineering from Duke University. He is a member and past Chair of the TRB Committee on Passenger Travel Demand Forecasting. 相似文献
T. Keith LawtonEmail: |
Lawrence Frank is an Associate Professor and Bombardier Chair in Sustainable Transportation at the University of British Columbia and a Senior Non-Resident Fellow of the Brookings Institution and Principal of Lawrence Frank and Company. He has a PhD in Urban Design and Planning from the University of Washington. Mark Bradley is Principal, Mark Bradley Research & Consulting, Santa Barbara California. He has a Master of Science in Systems Simulation and Policy Design from the Dartmouth School of Engineering and designs forecasting and simulation models for assessment of market-based policies and strategies. Sarah Kavage is a Senior Transportation Planner and Special Projects Manager at Lawrence Frank and Company. She has a Masters in Urban Design and Planning from the University of Washington and is a writer and an artist based in Seattle. James Chapman is a Principal Transportation Planner and Analyst at Lawrence Frank and Company in Atlanta Georgia. He has a Masters in Engineering from the Georgia Institute of Technology. T. Keith Lawton transport modeling consultant and past Director of Technical services, Metro Planning Department, Portland, OR, has been active in model development for over 40 years. He has a BSc. in Civil Engineering from the University of Natal (South Africa), and an M.S. in Civil and Environmental Engineering from Duke University. He is a member and past Chair of the TRB Committee on Passenger Travel Demand Forecasting. 相似文献
125.
This paper presents a fatigue design method for plug and ring type gas welded joints, which incorporates welding residual
stress effects. A non-linear finite element analysis (FEA) was first performed to simulate the gas welding process. The numerically
predicted residual stresses of the gas welds were then compared to experimental results measured using a hole drilling method.
In order to evaluate the fatigue strength of the plug and ring type gas welded joints, a stress amplitude (σ
a
)
R
taling the welding residual stress of the gas weld into account was introduced and is based on a modified Goodman equation
incorporating the effect of the residual stress. Using the stress amplitude (σ
a
)
R
, the ΔP-N
f
relations obtained from fatigue tests for plug and ring type gas welded joints having various dimensions and shapes were
systematically rearranged into (σ
a
)
R
-N
f
relations. It was found that the proposed stress amplitude (σ
a
)
R
could provide a systematic and reasonable fatigue design criterion for the plug and ring type gas welded joints. 相似文献
126.
Y. H. Rim J. H. Moon G. Y. Kim S. D. Noh 《International Journal of Automotive Technology》2008,9(6):719-728
Automotive general assembly requires many manual assembly operations to be carried out by human workers. Ergonomic analysis
is an important part of the design and evaluation of products, jobs, tools, machines and environments for safe, comfortable
and effective human functioning. Most recent researches have involved the evaluation of working conditions to prevent work-related
musculoskeletal disorders. The majority of previous research on automotive companies has mainly considered the results of
ergonomic analyses such as RULA (Rapid Upper Limb Assessment), REBA (Rapid Entire Body Assessment) and OWAS (Ovako Working
Posture Analysis System). Analysis of static posture including reachability, clearances for arm, hand and tool has also been
used to evaluate working conditions. However, in addition to static posture analysis, a biomechanical analysis in dynamic
conditions should also be conducted. There are no integrated frameworks or standard schema for ergonomic analysis using digital
human models in digital environments. The purpose of this paper is to propose a new framework for the evaluation of working
conditions by ergonomic and biomechanical analysis using digital models based on XML standard schema, including: products,
processes, manufacturing resources and human workers. This paper presents the analysis results using the proposed framework
for automotive general assembly operations. We propose a new framework for the evaluation of the assembly operations and their
environments. Then we apply a digital human model to the dynamic simulation of general automotive assembly operations based
on standard schemas in XML and PPRH (Product, Process, Resource and Human). Using PPRH information based on a standard XML
schema to analyze the ergonomic and biomechanical results, the engineer can visualize, analyze and improve assembly operations
and working environments in automotive general assembly shops using digital models. 相似文献
127.
E. Zervas 《International Journal of Automotive Technology》2008,9(6):641-647
This work studies the impact of five parameters: CO and HC engine-out emissions, space velocity, average value and profile
of exhaust temperature, on Diesel CO and HC tail-pipe emissions. The first part of this work is conducted on a reactor and
shows that both HC and CO light-off temperature increases with CO and HC input concentration. CO and HC initial concentration
influence the adsorption/desorption capacities of HC only at high temperatures. Space velocity also influences CO and HC conversion
efficiency. The second part of this work studies the impact of different combinations of HC and CO engine-out emissions on
CO and HC conversion and tail-pipe emissions in the case of New European Driving Cycle. This part proposes that a Diesel oxidation
catalyst must be mainly studied at the Urban Part of NEDC, as the CO and HC conversions are very high at the extra-urban part
of NEDC. CO and HC conversion efficiencies are also dependent on exhaust temperature and catalytic volume. In the case of
two different profiles of exhaust temperature with the same average temperature, CO and HC conversion efficiency is lower
in the case of the smoother profile. 相似文献
128.
Depth of field effects in laser sheet imaging were considered for droplet sizing of a pre-swirl spray. A pre-swirl spray is
formed before the hollow-cone type main-swirl spray from a D.I. gasoline injector, and shows transient characteristics with
high axial velocity. A microscopic imaging technique was applied to obtain high spatial resolution LIF tomograms of the pre-swirl
spray. A 1 mm thick Nd:YAG laser sheet was used as a light source to make the LIF tomograms that were imaged using a high-resolution
CCD camera. The droplet sizing of the pre-swirl spray was carried out using an image processing technique. In the image processing
procedure, the laser sheet-straddling large-sized droplets were carefully taken into account to remove the errors caused by
the depth of field effects from the limited thickness and the energy distribution of the laser sheet. The mean intensity of
the individual droplets and the line profile of the LIF signal around the droplet edge were inspected to screen the laser
sheet-straddling large-sized droplets. In order to consider the effects of the size-dependent LIF signal intensity, the size-classified
or ensemble-averaged mean intensity of the individual droplets was introduced. The mean droplet sizes such as AMD and SMD
were calculated using only screened droplets, and they slightly increase before considering the depth of field effects. 相似文献
129.
This study summarizes engine speed and load effects on HC species emissions from premixed charge compression ignition (PCI)
and conventional diesel combustion, and it evaluates diesel oxidation catalyst (DOC) formulations on a gas flow reactor for
the purpose of diesel particulate filter regeneration or lean NOx trap desulfation. HC emissions are sampled simultaneously by a Tedlar bag for light HC species and by a Tenax TA™ adsorption
trap for semi-volatile HC species, and they are analyzed by gas chromatography with a flame ionization detector. The bulk
temperature and residence time during combustion are key parameters that are important for understanding the effects of speed
and load on engine-out HC emissions. The degree of post-flame oxidation is higher in PCI than in conventional combustion,
and it is increased for PCI with a higher speed and load, as indicated by a lower fuel alkanes/THC ratio, a higher alkenes/fuel
alkanes ratio, and a higher methane/THC ratio. Ethene and n-undecane are two representative HC species, and they are used
as a surrogate mixture in the gas flow reactor to simulate PCI and conventional combustion with in-cylinder post fuel injection.
Among the three DOC formulations tested, the catalyst with constituent precious metals of platinum and palladium (PtPd) showed
the best light-off performance, followed by PtPd with an addition of cerium dioxide (PtPd+CeO2), and platinum (Pt), regardless of exhaust compositions. Conventional combustion exhaust composition shows a lower light-off
temperature than that of PCI, regardless of catalyst formulation. 相似文献
130.
This work presents an experiment on the relationships between subjective and objective evaluations of vehicle handling. Ten
cars were examined objectively in several open-loop driving dynamics manoeuvres and subjectively by test persons in typical
traffic situations. Results are extracted from a stationary test (the Slowly Increasing Steer Test), and a dynamical test
(the Frequency Response Test). The subjective measurements are obtained from drivers on a rural road course via a questionnaire,
which was developed to separately investigate the quantity level perception, the so-called “Niveau”, and the more qualitative
“Liking”. These subjective “measurements” are embedded into a two-channel definition of “Steering Comfort” as a genus for
“Steering Discomfort” and “Character”. The article concentrates on developing a statistical method for the consideration of
correlations amongst the subjective/objective data. In doing so, the variance in example subjective Niveau ratings can be
significantly explained by several objective quantities. Indicators for co-domains of validated discomfort characteristics
and hints for endeavouring character Liking ranges are detected. 相似文献